Polymers and oligomers having carbamate functional groups have been used in a variety of curable compositions. Carbamate-functional acrylic polymers are described, for example, in U.S. Pat. No. 5,356,669 and WO 94/10211. These can be prepared by addition polymerization of carbamate-functional acrylic monomers or by transcarbamylation of a hydroxy functional acrylic with an alkyl carbamate. Carbamate-functional polyesters, prepared by transcarbamylation of a hydroxy-functional polyester, are described in JP 51/4124.
Polyesters are widely used in curable compositions such as coating compositions. These resins offer many beneficial properties, such as good durability, good dispersibility in aqueous systems through incorporation of appropriate ionic or nonionic stabilizing groups, impact resistance, good adhesion, and other physical properties such as stress release. One area of concern with polyester resins for curable compositions has been the incorporation into the resin of sufficient levels of functional groups to achieve the desired cure performance. Hydroxyl groups are commonly used as functional groups in curable compositions, but polyester resins with pendant hydroxyl groups are difficult to prepare since any pendant hydroxyl groups would be consumed by reaction with acid groups during formation of the polyester. Hydroxyl functional groups are usually incorporated onto polyester resins by the use of polyol capping agents like trimethylol propane resulting in terminal OH groups, but no pendant OH groups. Such resins provide only limited crosslink density upon cure. The crosslink density may be increased somewhat by using branched polyesters, which are prepared by the incorporation of trifunctional or higher functional polyols or polyacids in the polyester reaction mixture. However, the degree of branching is often limited due to gelation. Low crosslink density in curable polyester resin systems must often be compensated for by using higher molecular weight resins that more closely resemble thermoplastic compositions than thermoset compositions.
Carbamate-functional polyesters are described in JP 51/4124. This reference describes the preparation of polyesters having carbamate terminal groups by the transesterification of a typical hydroxy-functional polyester with an alkyl carbamate.
Accordingly, the present invention is directed toward a new method of preparing polyester polymers or oligomers having pendant carbamate groups. It is also desirable to prepare polyester polymers or oligomers while avoiding the formation of side products that could reduce the purity of the final polyester.